1. Field of the Invention:
The present invention relates to a process for the preparation of coarse aqueous plastic dispersions.
2. Discussion of the Background:
In the context of the present invention, coarse dispersions are taken to mean dispersions whose particles have a volume-average diameter (dv) in the range of 120 to 400 nm, in particular 130 to 300 nm. Dispersions of this type are still capable of flowing even after concentration to a solids content of &gt;55% by weight and are therefore regarded as having a low viscosity.
Processes in which an anionic emulsifier, a nonionic, water-soluble or water-dispersible, polymeric auxiliary and a water-soluble salt are present during the batch polymerization have already been disclosed in the prior art.
DE-OS 2,165,410 discloses gelatin, caustisized starch, polyacrylamide, poly(ethylene oxide) (poly(ethylene glycol)), poly(vinyl alcohol), and poly(vinyl methyl ether) as polymeric auxiliaries. The polymerization is preferably carried out in the presence of a water-soluble salt containing a divalent metal ion. However, the process disclosed in DE-OS 21 65 410 requires comparatively long polymerization times. In contrast, short polymerization times are desirable as far as the economic efficiency of the process is concerned. It is generally true that in this method the larger the particle size required, the longer the polymerization time.
EP-OS 115,468 discloses ethylene oxide/propylene oxide copolymers and poly(ethylene oxide) as polymeric auxiliaries. In this method, it is necessary to add phosphate esters as anionic emulsifiers. However, it is desirable to avoid the use of phosphates for environmental considerations. Potassium persulfate and ammonium persulfate act as both polymerization initiator and salt.
German Pat. No. 2,432,983 discloses a poly(ethylene oxide) having a molecular weight of 20,000 as a polymeric auxiliary which should only be added when at least 50% by weight of the monomers have reacted. Otherwise, the polymerization rate is considerably slowed, and crust formation occurs. The disclosed salt is potassium chloride. This method exhibits the disadvantage that a viscosity maximum is observed at a monomer conversion of about 20 to 40% by weight in the case of a high monomer/water ratio, and the process is therefore difficult to control. A high monomer/water ratio is desirable for economic reasons. In a comparative experiment, the poly(ethylene oxide) having the molecular weight 20,000 is added before the polymerization.
The poly(ethylene oxides) and ethylene oxide/propylene oxide copolymers of the outlined prior art are obviously unbranched.
DE-OS 2,222,176 discloses, inter alia, poly(vinyl alcohol), partially hydrolized poly(vinyl acetate), poly(vinylpyrrolidone), poly(ethylene oxide), poly(propylene oxide) and ethylene oxide/ propylene oxide copolymers as polymeric auxiliaries which are added before the polymerization. Products of the reaction of a poly(ethylene oxide) and a .music-flat.polyepoxide" obtained by reacting epichlorohydrin with a polyhydric phenol are preferred. In the examples, a "poly(oxyethylene) glycol compound" having the molecular weight 20,000 is particularly emphasized. Further details of the structure of this compound and on a process for its preparation are not given.
If details in this regard are taken into account from the description in DE-OS 2,222,176, the preparation by reacting a poly(ethylene oxide), i.e. a bifunctional alcohol, with a trifunctional or polyfunctional epoxide is ruled out since the resulting crosslinked poly(ethylene oxide) compound would not meet the condition of adequate water solubility and would therefore be unsuitable as a polymeric auxiliary. It is concluded that the poly(ethylene oxide) compound must be obtained by reacting a poly(ethylene oxide) with a bifunctional epoxide and is essentially unbranched. For example, the poly(ethylene oxide) compound could have been obtained by reacting a poly(ethylene oxide) having the molecular weight 5,000 with a diepoxide at a molar ratio of four to three. The poly(ethylene oxide) compound has a molecular weight distribution of varying width, depending on the width of the molecular weight distribution of the poly(ethylene oxide) employed. In Example 4 of DE-OS 2,222,176, a series of experiments using various electrolyte additives is reported, and it is disclosed that the electrolyte additive (salt additive) in addition to the potassium peroxodisulfate has no effect on the particle size.
In addition, it is difficult to prepare coarse dispersions which are at the same time substantially free of coagulate using the process of DE-OS 2,222,176.
German Pat. Nos. 1,770,934 and 2,002,094 disclose batch processes for the preparation of coarse, carboxylated synthetic rubber dispersions which do not correspond to the present process, inasmuch as the polymeric auxiliary is not present during the polymerization, but is only added after the polymerization. The auxiliary is an oxidized poly(ethylene oxide) and is added in comparatively large amounts (for example 0.1 part by weight, based on 100 parts by weight of solid). This method is disadvantageous in that an additional process step is necessary after the polymerization, that the electrolyte stability of the dispersions is inadequate for specific applications, and that the dispersions have a rather have a rather broad particle size distribution which does not always meet the demands of industry. A typical dispersion has a non-uniformity, U, of 0.18, where U is defined as follows: ##EQU1## in which dv is the volume-average diameter; and dn is the number-average diameter.
The average diameters are determined by an electron-microscopic measurement method in combination with DIN 66 141. Dispersions having a narrow, monomodal or a less narrow, bimodal particle size distribution cannot be prepared by this method. In contrast, a narrow, monomodal or a less narrow, bimodal distribution is desired. In the latter case, the two groups of particles should differ essentially with respect to their size (see, e.g., P. H. Johnson et al., Rubber World, 139, 227 (1958) and J. H. Waterman et al., J. Inst. Rubber Ind., 1, 168 (1967)). In addition, the particles produced by this method are generally non-spherical, i.e. they have an irregular shape, which gives rise to a disadvantageous effect on the shelf life of the dispersions in that the viscosity increases in an undesired way on storage.
DE-OS 2,323,547 discloses another batch process for the preparation of coarse synthetic rubber dispersions, in which the polymeric auxiliary is not added until after the polymerization and which suffers from similar disadvantages. The auxiliary is added in comparatively large amounts (0.01 to 5 parts by weight, preferably 0.02 to 1.0 parts by weight, based on 100 parts by weight of solid). A typical polymeric auxiliary is an ethoxylated castor oil having the molecular weight 9,600.
Thus, there remains a need for a process for the production of coarse, aqueous plastic dispersions which does not suffer from the above-mentioned shortcomings.